Outdrive units are used with in-board motored vessels to transmit the rotational motion of the in-board motor within the vessel to rotational motion of a propeller outside the vessel and immersed in the surrounding water. The in-board motor is coupled to the outdrive unit by a drive shaft having a conventional U-joint at each end connected to a yoke of the motor at one end and a yoke of the outdrive unit at the other. The drive shaft passes from the motor through the transom at the rear of the vessel, through a mounting plate attached to the transom, through a drive shaft bell housing assembly attached to the plate via a gimbal ring, and to the outdrive unit. The bell housing assembly is attached to the plate via the gimbal ring at an upper steering bearing and a lower steering bearing that are arranged at vertical locations to accommodate right and leftward movement of the outdrive unit vis-a-vis the transom and, thus effect steering of the vessel. The degree to which the outdrive unit is immersed into the water, and the placement of the outdrive into a retracted portion for transporting, i.e., the vertical placement of the outdrive unit vis-a-vis the transom via a horizontal tilt axis of the gimbal ring, is controlled by a hydraulically operated tilt ram mounted between the outdrive unit and gimbal ring.
The conventional steering system used with the outdrive unit comprises a manually operated steering control, such as a steering wheel and the like, that transmits a steering command via a cable mechanism to a hydraulically operated steering control valve mounted within the vessel adjacent to and parallel with the transom. A piston extends from the control valve and is connected at one end to a tiller lever that extends through an opening in the transom, through the mounting plate and is connected at an opposite end to the gimbal ring. The control valve effects the inward and outward movement of the piston parallel to the transom according to respective leftward and rightward movement of the steering wheel. The inward and outward movement is translated by the tiller lever to steering movement of the outdrive unit.
When the outdrive unit is not operating with the motor lowered into the water, the weight of the outdrive unit, due to the placement of the unit rearward of the transom, imposes a rearwardly directed tension force on the upper steering bearing from the fulcrum action of the tilt rams. A frontwardly directed compression force is also imposed on the lower steering bearing. During the operation of the outdrive unit to propel the vessel, the tension force imposed on the upper bearing is increased due to the addition of a frontwardly directed dynamic force that is imposed on the bottom of the outdrive unit from the propulsion action of the propeller in the water. This static and dynamic tension force is unchecked by any structural member attaching the outdrive unit to the transom and frequently, if not usually, results in the failure of the upper steering bearing.
Due to the construction of the gimbal ring and the mounting plate and their relation to the outdrive unit, it is difficult to accommodate the destructive action of the tension force on the upper bearing by constructing a mounting plate having a structurally reinforced upper portion. The construction of such a reinforced mounting plate is precluded because of the need to keep the upper portion of the plate relatively small to accommodate the retracted positioning of the outdrive unit during transport. With respect to the lower steering bearing, the compression force that is imposed has not resulted in the same type of destructive action found in the upper bearing due in part to the nature of the lower portion of the mounting plate to better accommodate compressive forces. Additionally, unlike the upper portion of the plate, the lower portion can be structurally reinforced without impairing the vertical mobility of the outdrive unit.
It has been discovered that the efficiency of existing outdrive units, i.e. , gains in speed and acceleration, can be obtained by extending the outdrive unit a further distance rearward from the transom via the use of an extension unit. Outdrive extension units extend the outdrive unit approximately 8 to 9 inches rearward of the transom. The extension unit operates to increase the efficiency of the outdrive unit by revising the placement of the propeller relative to the vessel, namely, by placing the propeller in cleaner water less disturbed by the vessel hull, and improving the depth and attitude at which the propeller can be immersed in the water. This increases the action of the propeller against the water, thereby, increasing the speed of the vessel.
However, the use of an extension unit with the outdrive unit aggravates the existing problem with respect to the load carried by the upper steering bearing by approximately doubling both the static and dynamic tension forces that are imposed on the upper steering bearing. This loading force results in increased incidents of upper bearing failure.
It is, therefore, desirable that an apparatus be constructed that will counteract and minimize the static and dynamic tension forces imposed on the upper steering bearing used to connect an outdrive unit to its mounting plate to minimize damage caused by such forces and, thereby enhance the life of the upper steering bearing. It is desirable that the apparatus be capable of use with both existing outdrive units and with outdrive units incorporating an outdrive extension unit without significant modification and in a manner that facilitates easy installation. It is desirable that the apparatus be capable of minimizing the damage caused to the upper steering bearing in a manner that does not affect the vertical movement of the outdrive unit and does not adversely affect the efficiency or performance of the outdrive unit.